The present invention relates to a vacuum welding chamber intended for the butt welding, by an electron beam, of two elongate pieces which extend in their longitudinal direction at the centre of this welding chamber. Electron beam welding has appreciable advantages because it is suitable for pieces of high thickness, because it permits rapid and automated welding operations and because it does not require a heat treatment, but it requires the installation of a leak-tight envelope for creating an enclosure in which it is possible to create the vacuum essential for the welding operation. Now, in the case of elongate pieces of large dimensions, the enclosure created in this way has a very large volume which it is time-consuming and expensive to re-evacuate for each welding operation after it has been returned to atmospheric pressure as a result of introducing a new elongate piece into the welding chamber.